Reference is now made to FIG. 1 showing a block diagram of a touch panel sensing system 10. The system 10 includes a touch panel 12 formed of a plurality of intersecting conductive lines. The lines include a first set of parallel conductive lines 14 oriented to extend in a first direction and a second set of parallel conductive lines 16 oriented to extend in a second direction. In this implementation, the first and second directions are perpendicular to each other. The first and second lines are separated from each other by a thin insulating dielectric layer.
The system 10 further includes a control circuit 20. The control circuit 20 includes a digital processing circuit 22. The processing circuit 22 is configured to operate for the generation of AC drive signals (for example, a digital waveform) that is applied through a transmit (Tx) drive circuit 24 to the first set of lines 14 (which are referred to by those skilled in the art in this context as “force” lines). The drive signals are capacitively coupled to the second set of lines 16 (which are referred to by those skilled in the art in this context as “sense” lines) at each location wherein one of the lines 14 and one of the lines 16 intersect (pass over) each other. The control circuit 20 further includes one or more sensing circuits 26 coupled to the lines 16. The sensing circuit 26 functions as a charge-to-voltage (C2V) converter circuit operating, as is well known in the art, to convert the capacitance at the intersection of lines 14 and 16 to a voltage (the operation referenced to a common mode voltage Vcm). In operation of the touch panel sensing system 10, that line intersection capacitance is modulated by a proximately located object (such as, for example, the finger of a human being or a stylus), and thus the voltage generated at the output of the sensing circuit 26 will be indicative of the proximity of the object. The control circuit 20 further includes an analog accumulator circuit 28 that functions to accumulate over an accumulation time period the voltages output from the sensing circuit 26. At the end of the accumulation time period, an analog-to-digital converter (ADC) circuit 30 converts the accumulated voltage to a digital signal that is processed by the digital processing circuit 22 to extract information relating to the proximately detected object (such as, for example, a touch or hover location, a touch strength, etc.).
FIG. 2 illustrates operation of the system 10. The AC drive signal (Tx) in the form of a square wave signal is applied to a given one of the force lines 14 using the drive circuit 24. The sensing circuit 26 coupled to one of the sense lines 16 generates an output signal (C2V Out) dependent on the capacitance at the sensing location where the force line 14 and sense line 16 intersect. At the beginning of a sensing period S, the C2V Out signal is reset (reference 32) to the common mode voltage Vcm. Each accumulation time period includes a plurality of those sensing periods S, wherein each sensing period is associated with a given phase of the AC drive signal. The sensing circuit 26 then integrates (reference 34) charge to generate a voltage (reference 36) that is indicative of the charge at the sensing location. At the end of the sensing period S, the analog accumulator circuit 28 accumulates (reference 38) the voltage 36. Thus, the analog accumulator circuit 28 functions to sum the voltage 36 magnitudes for each sensing period S included within the accumulation time period. At the end of the accumulation time period, the accumulator output voltage is converted from the analog domain to the digital domain by the ADC circuit 30 for processing by the digital processing circuit 22.
Those skilled in the art recognize concerns with the noise phenomena known as electrical fast transient (EFT). This type of noise may generally be characterized having a high amplitude and a single frequency. Such noise can be present in a system 10 where it generates excessive charge coupled to the touch panel 12. Reference is made to FIG. 3 which illustrates operation of the system 10 with the EFT noise event indicated at reference 40. Because the sensing circuit 26 coupled to the sense lines 16 is a charge-to-voltage (C2V) converter circuit, the induced excessive charge of the EFT noise event will introduce an error (reference 42) in the C2V Out signal because that EFT-related charge is integrated (reference 44) by the sensing circuit 26. This error will then be accumulated by the analog accumulator circuit 28 (reference 46). This accumulated error offsets the accumulated value at the end of the accumulation time period (compare to FIG. 2) and introduces an error in the digital signal output by the ADC circuit 30
There is a need in the art to address concerns with errors introduced by electrical fast transient noise in the operation of capacitive touch panel sensing systems.